LibWeb: Start adding support for the <iframe> element! :^)

This patch introduces a bunch of things:

- Subframes (Web::Frame::create_subframe())
- HTMLIFrameElement (loads and owns the hosted Web::Frame)
- LayoutFrame (layout and rendering of the hosted frame)

There's still a huge number of things missing, like scrolling, overflow
handling, event handling, scripting, etc. But we can make a little
iframe in a document and it actually renders another document there.
I think that's pretty cool! :^)

Libraries/LibWeb/Frame.h

@@ -41,6 +41,7 @@ class PageView;
 
 class Frame : public TreeNode<Frame> {
 public:
+    static NonnullRefPtr<Frame> create_subframe() { return adopt(*new Frame); }
     static NonnullRefPtr<Frame> create(PageView& page_view) { return adopt(*new Frame(page_view)); }
     ~Frame();
 
@@ -64,6 +65,7 @@ public:
     void did_scroll(Badge<PageView>);
 
 private:
+    Frame();
     explicit Frame(PageView&);
 
     WeakPtr<PageView> m_page_view;